Senior FBI and CIA officials believe the American anthrax attacks are being carried out by US extremists probably not connected to the al-Qaida terror network, the Washington Post reported today.

A senior official was quoted in the Post as saying that "everything seems to lean toward a domestic source. Nothing seems to fit with an overseas terrorist type operation".

Authorities are considering many possibilities, the Post reports, including the associates of right-wing hate groups and people in the US sympathetic to Islamic extremist causes. The attacks have left three people dead.

New evidence of the bacteria emerged at Capitol hill in Washington today and at a growing list of government mail rooms.

Trace amounts of anthrax were detected in the offices of three congress members in the Longworth building, which has been closed for more than a week. Mail delivery on Capitol hill has been suspended even longer.

US health officials ordered anthrax testing at thousands more mail rooms across the Washington area but continued to insist that postal customers had very little risk of falling victim to biological terror. Concerns over the mail delivery system continues to increase with anthrax now confirmed at more than 10 postal facilities in the area, and health officials hoped to stop its spread with an escalation of testing and treatment.

"We're getting in front of the fire", said Dr Patrick Meehan of the Centres for Disease Control and Prevention. But he warned that thousands more people who work with the mail will need to begin taking preventive antibiotics. "It could be an astronomical number," he said.

Three weeks into the anthrax-by-post scare, officials said they would begin testing at between 2,000 and 4,000 sites that receive mail from the city's Brentwood postal processing facility, where two have workers died of anthrax.

The majestic Supreme court building also fell victim to the spreading bioterrorism scare, as officials ordered it closed for testing after a trace amount of anthrax was detected in an offsite mail centre.

Anthrax was also reported at a CIA mail facility, at Walter Reed army medical centre and a Washington post office - all of which receive mail from the Brentwood facility.

Officials emphasised that people who work full-time with mail are at a higher risk than postal customers, who only touch the letters meant for them.

For typical Americans, Dr Meehan said, the mail poses "an incredibly remote risk". Still, they recommended that people look at their mail closely and wash their hands after touching anything suspicious.

The funeral for Joseph Curseen Jr, a 47-year-old mail processor who worked at the Brentwood facility, was being held today. Mr Curseen, who had 15 years of service, died on Monday.

The funeral of his colleague, Thomas Morris Jr, 55, was yesterday. Mr Morris had worked for the postal service for 28 years and died last Sunday.